Meeting timing requirements is a critical portion of the design and operation of systems in a wide variety of technological fields. Computer chipmakers continually reduce the minimum size of devices while simultaneously increasing speed and complexity to improve performance. Each reduction in size and increase in speed demands a higher level of precision in the timing of the electrical pulses input and output by the chip.
A number of methods and devices have been developed for high-speed and high-precision timing testing. The devices, generally referred to as “Automated Testing Equipment,” come in a wide variety of forms.
Automated Test Equipment includes devices, frequently referred to as “Time Measurement Units,” for measuring time intervals and signal events. An example of a time measurement unit is found in U.S. Pat. No. 6,091,671, issued to Kattan, for a time interval analyzer, which is incorporated herein by reference.
A number of time measurement units are commercially available to measure time events and interpret data gathered from such measurements. These time measurement units are typically used in conjunction with some sort of measurement interface between the point of measurement (for example, an electronic circuit board connection) and the time measurement unit. The measurement interface, referred to as an Automated Testing Equipment (ATE) loadboard, is commonly connected to the time measurement unit via (typically coaxial) cables.
The cables, while providing a signal path from the testing equipment to the time measurement unit, are not ideal components. As the frequency of a signal increases, physical factors such as a cable's length and frequency response become substantially more important.
At extremely high frequencies, even the slightest deviations in the cables—deviations such as bends, kinks, or length variations—can have an adverse effect on the accuracy of time measurements. When the length of a signal pulse is many orders of magnitude smaller than the length of the cable it is sent through, a simple bend in the cable could delay the signal enough to render a time measurement useless. Since it is nearly impossible and not economical to eliminate all the causes of such problems, there must be compensation for the signal irregularities that the cables introduce.
In the prior art there can be found several attempts to address the problem of cable-induced signal irregularities through calibration of test equipment. One such example can be found in U.S. Pat. No. 6,032,107 issued to Hitchcock, entitled “Calibrating Test Equipment.” The calibration methods and apparatus taught in the Hitchcock patent rely upon usage of a test article measured with calibrated test equipment. The test article is measured using the uncalibrated equipment, and the resulting measurements are compared to those taken by the calibrated equipment. One of the deficiencies of this method, however, is that it does not address the situation where there is no previously calibrated equipment.
Additionally, the complex nature of many time measurement units can be the source of timing errors as well. Typically, a time measurement unit must be “armed,” or prepared, to take a measurement. The arming circuitry of the time measurement unit causes a delayed response to the arming signal. This arming latency can vary with temperature and other factors, and introduces another variable that must be taken into account in time measurement. Without knowledge of the arming latency, it can be very difficult to make accurate measurements because the exact time that the measurement is taken is indeterminate.